Method of producing rare earth oxyfluoride



Sept. 12, 1967 L. R. WOOD METHOD OF PRODUCING RARE EARTH OXYF'LUORIDE Filed April 28, 1964 2 Sheets-Sheet 1 Fig. 2

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United States Patent 3,341,437 METHOD OF PRODUCING RARE EARTH OXYFLUORIDE Lyle Russell Wood, P.O. Box 965, Apple Valley, Calif. 92307 Filed Apr. 28, 1964, Ser. No. 363,190 12 Claims. (Cl. 204-61) This invention relates to a composite of novel inorganic compounds containing oxygen and fluorine and a process and device for its production. In particular, it refers to a group of compounds containing the rare earth elements and oxygen and fluorine and an electrolytic process for its production.

The rare earth elements, as is generally known, comprise those elements with atomic numbers from 57 to 71 and include lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutecium. This group of transition elements within a larger transition element section on the periodic table have remarkably similar properties, always occur together in nature and are in most cases extremely difiicult to separate to any significant purity. These characteristics are occasioned by the fact that the diifering electron structure for the rare earth elements occurs within the antipenultimate electron shell while their ultimate and penultimate electron groups are approximately identical.

The rare earth metals are commonly extracted and purified. No fixed ratio of each element is of significance since their properties are so similar and they occur together in nature in varying proportions depending upon the location from which the mineral is obtained. The processing of the minerals does not change their ratios very much. Typically, lanthanum may be about 25-35% of the total weight of rare earth elements, cerium 45- 60%, praseodymium 612%, neodymium 3-6 Samarium and europium from very small amounts to a percent or two, and the other in small quantities.

In View of the known character of the rare earth elements, they will hereinafter "be referred to as a group, rare earth elements, and a composite of compounds containing the rare earth elements as cations will hereinafter be referred to as a rare earth compound.

The present invention has for its primary object the provision of a novel composite of compounds comprising the rare earth elements, fluorine and oxygen. A further primary object resides in a process for the economical manufacture of such compounds composite.

Another object of the invention resides in the process of forming oxyfluoride from rare earth fluoride and oxygen.

An additional object is inherent in the use of an elec trolytic process for the economical manufacture of rare earth oxyfluoride.

A still further aspect of this invention resides in the design and structure of the electrolytic cell to carry out the electrolytic process for the production of rare earth These and other objects will become more apparent from the following description. It has been found that a combination of the rare earth elements, in approximately the same ratio as they occur naturally, and oxygen and fluorine, Where the rare earth elements act as the cation, produces a composite of compounds having a high melting point, is resistant to attack by molten fluorides, resistant to oxidation, and resistant to attack by molten misch metal, i.e. the rare earth elements in their metallic state. This composite of compounds or rare earth oxyfluoride is rock-like, typically black, red, brown, or purple, and has a specific gravity of approximately 5.0 or

more. It is capable of being crushed and ground similar 3,341,437 Patented Sept. 12, 1967 ice to rock. Other chemical properties of rare earth oxyfluoride include resistance to attack by boiling concentrated hydrochloric and sulphuric acids. Due to its extremely high resistance to attack by known deleterious chemicals, particularly molten fluorides, rare earth and others, and the rare earth elements, rare earth oxyfluoride has considerable commercial value in the production of such compounds as misch metal and other reactive metals and in other chemical and metallurgical fields.

It has been found that rare earth oxyfluoride can be conveniently economically produced electrolytically utilizing an electrolyte consisting of rare earth fluoride in an atmosphere containing oxygen. The rare earth fluoride is a conventional composite of compounds having the rare earth elements as cations. Whilepurified or selectively separate fluorides of specific rare earth elements may be used, they do not normally present any advantage and are disadvantageous by reason of the great expense involved in effecting their separation and purification. Consequently, it is preferred to utilize rare earth fluoride as defined.

In order to facilitate the electrolysis, since rare earth fluoride has a rather high melting point, it is desirable to use a mixture of rare earth fluoride in combination with the fluorides of alkali metals and alkaline earth metals, such as sodium, potassium, lithium, barium or calcium. The use of such alkali and alkaline earth fluorides in combination with the rare earth fluoride reduces the melting temperature of the electrolyte to an economically feasible value for carrying out the electrolysis.

In order to cause the formation of the rare earth oxyfluoride an electrode means which passes an alternating current through the electrolyte is utilized. This alternating current causes alternate reduction and oxidation at the electrode means and this action generates the rare earth oxyfluoride at each electrode. With this formation of the rare earth oxyfluoride, under the direction of a direct current action, the rare earth oxyfluoride can be selectively collected.

However, a melt comprising merely a combination of fluorides will not carry a direct current for a very long period of time Without polarization. Accordingly, since a direct current perm-its rapid collection of the produced rare earth oxyfluoride it is desirable to add a small amount of rare earth oxide to the molten fluoride or fluorides in the electrolyte. The rare earth oxide ionizes within the melt and conducts the current within the cell. The rare earth oxide can be as the oxide itself or as rare earth carbonate, rare earth carbonate mineral, or rare earth fluorocarbonate mineral such as bastnasite. Since the rare earth oxide decomposes during the electrolysis it is necessary that it be added to the cell at approximately the rate it is being decomposed electrolytically.

The electrolysis need be carried out in an atmosphere containing oxygen in order that oxygen as dissolved in the molten electrolyte be available for formation with the rare earth fluoride to produce the rare earth oxyfluoride.

The details of the process for the formation of the rare earth oxyfluoride and the construction and operation of the electrolytic cell are more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout and in which:

FIGURE 1 is a view in vertical section through a typical electrolytic cell adaptable for the process of this invention;

FIGURES 2 through 10 are schematic diagrams of electrical circuits and electrode means adaptable to the electrolytic cell or series of cells for the formation of rare earth oxyfiuoride.

Referring to FIGURE 1 in which the numeral 10 designates generally an electrolytic cell comprising a part of this invention, the container or cell wall 12 is made of any suitable metallic material, such as iron, steel, or the like. While the interior of the cell is to be maintained at an extremely high temperature, the exterior of the cell wall 12 should not be well insulated thermally in order to permit a high heat loss through the wall. Into the container 12 is introduced a mixture of the rare earth fluoride and the one or more fluorides of alkali and alkaline earth metals together with rare earth oxide. The fluorides and oxide are ground particles of convenient size.

During the normal operation of the cell a large alternating current is impressed between electrodes 14 and 16 through the electrolyte which comprises the fluorides and oxide in molten or liquid form, such as at 24. These AC electrodes 14 and 16 are made of any suitable size and conventional material, such as carbon or graphite or the like. The high alternating current generates heat and the molten fluorides and oxide are maintained in a liquid state by the expenditure of electrical energy. While this extremely high temperature within the molten electrolyte adjacent or nearby the alternating current electrodes maintains the electrolyte in a liquid phase, the high heat loss through the container wall 12 prevents melting of the particles of fluorides and oxide adjacent the interior of the container wall 12, as at 22. These unmelted fluorides and oxide 22 act as a skull to surround the molten electrolyte 24 and protect the cell wall from being attacked by such molten electrolyte. It is the combination of the high temperature within the molten electrolyte and the high heat loss through the container 12 which causes the skull of unmelted electrolyte 22 to completely surround the molten pool of electrolyte 24. As such, the container 12 acts merely as a mechanical support for the skull 22.

In order to form the skull 22 at the beginning of operating the cell 10, the electrodes 14 and 16 are placed in contact with the wholly unmelted electrolyte and the electrodes are shunted at the contact with the solid electrolyte by conventional means such as a piece of carbon or graphite. Due to the extremely high current passing through the shunting means the heat produced transforms the solid fluorides and oxide into a molten pool. Once sufficient fluorides and oxide have been melted, the cell is ready for normal operation.

The alternating voltage applied across the electrodes 14 and 16 is generally about 46 volts but can be as high as 10-12 volts and sometimes even higher, which voltage produces a high alternating current between these electrodes. The alternating current not only generates the heat necessary to keep the electrolyte in the desired molten state, but also causes the alternate reduction and oxidation at each of the electrodes and by this action generates the rare earth oxyfluoride at each electrode.

The rare earth oxyfluoride which forms tends to distribute itself throughout the electrolyte. The agitation in the cell from thermal convection, anode gassing, and the passage of large electric currents tends to keep the oxyfluoride from settling out. In order to collect the rare earth oxyfluoride so formed, a direct current is impressed between the anode 18 and the cathode 28. Due to the action of this direct current the rare earth oxyfluoride migrates to the cathode and builds up in a continually thickening deposit 26. Direct current voltages of about 3-6 volts have been found to be most appropriate but voltages outside this range may be used in some cases. The cathode 20 can be made of any conventional material such as steel, carbon or graphite in view of recognized cathodic protection and the added fact that the cathode is soon covered with at protecting layer of rare earth oxyfluoride 26. The anode 18 is most easily made of carbon or graphite.

The rare earth oxide is necessary in the electrolyte because the cell could not otherwise carry the direct current without polarization which would cause the current to stop flowing. The ionization of the rare earth oxide permits the passage of the direct current, but the oxide is electrolytically decomposed in so doing. This decomposition makes the oxygen anion available at the anode which unites with the carbon of the anode to form carbon monoxide producing a gassing effect as at 28. The rate of consumption of the anode is very small for the amount of rare earth oxyfluoride produced. A loose fitting cover (not shown) on the cell conserves heat and prevents rapid circulation of air around the hot electrodes above the molten electrolyte. Some air is necessary in the cell, however, to provide the oxygen for the formation for the rare earth oxyfluoride.

When the rare earth oxide portion of the molten electrolyte is depleted the direct current stops flowing. If an electrical arrangement for the electrode means is used in which the alternating current electrodes are used as the anode, such as in FIGS. 59, the alternating current will also stop flowing, or at least reduce considerably, when the oxide is depleted. Accordingly, it is necessary to occasionally add rare earth oxide to the molten electrolyte.

When the rare earth fluoride portion of the molten electrolyte is depleted, the action of the cell in forming rare earth oxyfluoride slows to an uneconomical rate. Therefore rare earth fiuoride must be continuously added to the molten electrolyte at approximately the same rate as the oxyfluoride is formed.

The rare earth oxyfluoride migrates to the cathode and collects around it in a massive accumulation. The cathode can be removed from time to time with the rare earth oxyfluoride adhering to it, a fresh cathode being inserted in its place. When the mass on the removed cathode has cooled, the rare earth oxyfluoride can be broken off and the cathode reused. The extraction of the cathode may be done without interrupting the alternating current.

FIGURES 2 through 10 illustrate various adaptable electrical arrangements whereby alternating and direct currents can be applied to the electrolyte by the cell. In each figure, AN designates an anode, CA designates a cathode, and AC designates an alternating current electrode.

FIGURE 2 illustrates an electrical arrangement for the cell having a separate DC and separate single phase AC connections.

FIGURE 3 shows two single phase alternating current circuits and separate direct current circuits. In a similar manner any number of alternating current circuits may be used.

FIGURE 4 illustrates utilization of a three phase alternating current circuit with a separate direct current. The alternating current electrodes can be connected to any poly-phase source of proper voltage and current capabilities. The number of anodes is independent of the number of phases used, and any number may be used so long as there is at least one.

FIGURE 5 shows a single phase alternating current arrangement with the alternating current electrodes serving as anodes. This connection, however, does use a separate direct current supply as shown.

FIGURE 6 illustrates an electrical arrangement using a three phase alternating current power source with separate direct current utilizing the alternating current electrodes as the anodes. Center tap connections are used to prevent magnetization of the transformer cores by the 'DC current flowing through them.

FIGURE 7 illustrates a single phase AC supply with a rectifier supplying the negative charge for the cathode. The AC electrodes also act as the anodes.

FIGURE 8 illustrates a three phase AC supply with the AC electrodes serving as the anodes. Three rectifiers, one to each phase lead, create the negative charge for the cathode.

FIGURE 9 illustrates a three phase supply with the AC electrodes serving as anodes. The direct current is supplied by a single rectifier connected to the mid-point at t transformer windings.

FIGURE 10 illustrates a series connection of twocells to which other cells could be added similarly in order to have any number of cells operating in the line. The primaries of the transformers may be connected in series or in parallel and the alternating current circuits may be poly-phased. The advantage of connecting the cells in series results from the ability to supply a certain amount of direct current power more economically at higher voltages than at the low voltage required by each cell. In FIGURE 10 the series connection is eifected by insulating the transformers in the alternating current circuits sothat there is no path for direct current from cell to cell through the alternating current circuits.

The best temperature of operation has been found to be approximately 900-1000 C. but cooler or hotter temperatures may be used. In general, the higher the temperature the more voltage may be applied to and current may be passed through the cell, both alternating and direct. Actual experimental work has been carried out in a range of cell sizes from a 6 in. diameter to a 15 in. diameter cell.

Examples.-In a cell 14 in. in diameter and 16 in. deep, connected similarly to the diagram shown in FIG. 3, two pairs of alternating current electrodes of 1 in. diameter graphite immersed about 2 in., were operated with 10 volts and 180 amperes between each pair of electrodes. The anode was of .1 in. diameter graphite immersed 2 in. and the cathode was of 1 in. diameter steel, immersed 4 in. Four and eight tenths volts, 70 ampere direct current was used. The electrolyte initially comprise-d the following mixture by weight: 67% rare earth fluoride, 17% barium fluoride, and 16% lithium fluoride. Suflicient electrolyte in appropriate sized particles was introduced to fill the cell and about 600 grams per hour of rare earth fluoride was periodically added to maintain the electrolyte composition approximately constant. Bastnasite-was periodically introduced into the cell at the rate of about 160 grams per hour in order to maintain a satisfactory conductivity for the direct current. The temperature of the electrolyte was maintained at about 900-950 C. and during the electrolysis insulation was maintained by an electrolyte skull similar to that illustrated in FIG. 1. The cell produced approximately 600 grams of rare earth oxyfluoride per hour. An analysis of a specimen of the rare earth oxyfluoride indicated approximately 4% oxygen by weight.

In a cell approximately 6 in. in diameter and 6 in. deep of machined graphite, a three phase alternating current similar to that illustrated in FIGS. 6, 8 and 9 was supplied to three /2 in. carbon electrodes spaced equilaterally on a 3 in. diameter circle, with a A in. diameter molybdenum cathode in the center. Alternating currents of between 4 and 6 volts and up to 50 amperes were used. Direct currents of 25 amperes at 3 to volts were typical. The temperatures of the cells were maintained about 8001000 C., and the following combinations of compounds were initially introduced as the electrolyte (percentages are by weight):

Percent (a) Rare earth fluoride 65.7 Barium fluoride 10.8 Lithium fluoride 13.5 Rare earth oxide 110 (b) Rare earth fluoride 67 Barium fluoride 17 Lithium fluoride 16 (c) Rare earth fluoride 77.3 Barium fluoride 12.7 Lithium fluoride 10 (d) Rare earth fluoride 60.8 Barium fluoride Q 12.3 Lithium fluoride 26.9

(e) Rare earth fluoride 67.6 Barium fluoride 17.4 Lithium fluoride 14.6 Calcium fluoride 0.4

(1?) Rare earth fluoride 69.6

Additional rare earth fluoride was periodically introduced into the electrolyte in about the same weight quantity as the rare earth oxyfluoride was produced. Further, sufficient rare earth oxide was periodically added to the electrolyte in order to maintain the desired conductivity of the electrolyte for the direct current. In. each of the above examples of the electrolyte composition, rare earth oxyfluoride was produced and collected at the cathode.

Moreover, any combination of the rare earth, alkali and alkaline earth fluorides containing at least 40% by weight of the rare earth fluoride which produces a mixture melting at about 8004000 C. will serve appropriately as the electrolyte. Conversely, initial electrolytes such as 57% by weight potassium fluoride and 43% by weight barium fluoride do not produce rare earth oxyfluoride regardless of the length of time and the amount of rare earth oxide introduced since the electrolyte does not contain rare earth fluoride.

While it is not intended to be bound by any particular theoretical explanation for the production of the composite of compounds containing the rare earth elements, oxygen and fluorine by the process and electrolytic cell disclosed herein, it is believed the following correctly describes the chemical action taking place during the present process. Oxidation and reduction are terms applied to the valence change of elements, oxidation being the increasing of positive charge of the ion and reduction being the decreasing of the positive charge of the ion. This does not necessarily involve the element oxygen, and it can be brought about by a chemical reagent or by an electric current.

During the passage of alternating current through the molten fluoride electrolyte, the lanthanide elements are reduced at the electrode which is momentarily cathodic so that a film of the rare earth elements in their elemental form is created. During the next half cycle, when that electrode is not cathodic, but is in fact anodic, the rare elements which were deposited react with the fluoride melt and the oxygen which is dissolved in it to form the rare earth oxyfluoride.

The electrolyte of the present invention is composed of fluoride compounds of various metallic elements which do not contain oxygen. The rate of formation of the rare earth oxyfluoride appears to be at a faster rate than is accountable for by the rate of addition of oxygen in the form of rare earth oxide. Further, decomposition of the rare earth oxide is observed in gassing at the anode so that not much of its oxygen unites with the lanthanide elements and fluorine. Accordingly, since the only other source of oxygen in the electrolyte could be that which is dissolved, it is presumed the oxygen originates from the air above the molten electrolyte.

Rare earth oxyfluoride is prepared for use by crushing and pulverizing it and pressing it into the desired shape. This shape when heated to about 1100 C. becomes thickly pasty, probably due to the small amount of contained fluorides in the product. Continued heating at this temperature, or higher, for a day or to causes the form to set into a hard mass. The material may be used by itself or as a lining for any other material which it is desired to protect.

It will be understood that modifications in the invention will be evident to those skilled in the art and therefore it is not intended that the invention be limited to the details given herein but may be modified within the scope of the appended claims.

What is claimed as new is as follows:

, 1. The process of producing rare earth oxyfluoride which comprises an alternating current electrolysis of (1) a molten mixture of rare earth fluoride and one or more other fluorides selected from the group consisting of sodium fluoride, potassium fluoride, lithium fluoride, barium fluoride, and calcium fluoride and (2) molten rare earth oxide in an atmosphere containing oxygen.

2. The process of claim 1 in which said mixture includes at least 40 parts by weight rare earth fluoride.

3. The process of producing rare earth oxyfluoride which comprises an alternating current electrolysis of (1) a mixture of rare earth fluoride and one or more other fluorides selected from the group consisting ofsodium fluoride, potassium fluoride, lithium fluoride, barium fluoride and calcium fluoride, said rare earth fluoride comprising at least 40% by eight of said mixture, and (2) rare earth oxide in an atmosphere containing oxygen, said electrolysis carried out at a temperature of about 900-1000 C.

4. The process of forming rare earth oxyfluoride consisting of electrolyzing a melt which comprises (1) rare earth fluoride and (2) rare earth oxide in an atmosphere containing oxygen, said electrolyte being subjected to a continuous alternating current.

5. The process of preparing a composite of inorganic compounds containing the rare earth elements, fluoride and oxygen which comprises electrolyzing by an alternating current a melt of (l) rare earth fluoride, (2) one or more metal fluorides having a higher decomposition voltage than rare earth fluoride, and (3) rare earth oxide in an atmosphere containing oxygen, said melt maintained I at a temperature of about 9001000 C.

6. The process of forming an inorganic compound containing the rare earth elements, fluorine and oxygen which comprises an alternating current electrolysis of (1) a molten mixture of by weight approximately 67% rare earth fluoride, 17% barium fluoride and 16% lithium fluoride and (2) molten rare earth oxide in an atmosphere containing oxygen.

7. The process of forming rare earth oxyfluoride which comprises an alternating current, molten bath electrolysis of (l) a mixture of by weight of approximately 61% rare earth fluoride and 39% sodium fluoride and (2) rare earth oxide in an atmosphere containing oxygen.

8. The process of producing rare earth oxyfluoride which comprises impressing an alternating current through molten rare earth fluoride in an atmosphere containing oxygen.

9. An electrolytic process for producing rare earth oxyfluoride which comprises impressing an alternating current through an electrolyte of molten rare earth fluoride and dissolved oxygen.

10. The process of producing rare earth oxyfluoride which comprises an alternating current electrolysis of a melt comprising (1) rare earth fluoride, (2) one or more fluorides selected from the group consisting of alkali fluorides and alkaline earth fluorides and (3) rare earth oxide in an atmosphere containing oxygen.

11. A process for forming and collecting rare earth oxyfluoride which comprises impressing an alternating current and a direct current through a melt of rare earth fluoride and rare earth oxide in an atmosphere containing oxygen.

12. The process according to claim 10 wherein a direct current is impressed through said melt for collecting the formed rare earth oxyfluoride.

References Cited UNITED STATES PATENTS 3,185,652 5/1965 Kleber et al 106-55 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

8. THE PROCESS OF PRODUCING RARE EARTH OF OXYFLUORIDE WHICH COMPRISES IMPRESSING AN ALTERNATING CURRENT THROUGH MOLTEN RARE EARTH FLUORIDE IN AN ATMOSPHERE CONTAINING OXYGEN. 